William Stallings Computer Organization and Architecture7th Edition

Chapter 3System Buses

Program Concept• Hardwired systems are inflexible• General purpose hardware can do

different tasks, given correct control signals

• Instead of re-wiring, supply a new set of control signals

What is a program?• A sequence of steps• For each step, an arithmetic or logical

operation is done• For each operation, a different set of

control signals is needed

Function of Control Unit• For each operation a unique code is

provided—e.g. ADD, MOVE

• A hardware segment accepts the code and issues the control signals

• We have a computer!

Components• The Control Unit and the Arithmetic and

Logic Unit constitute the Central Processing Unit

• Data and instructions need to get into the system and results out—Input/output

• Temporary storage of code and results is needed—Main memory

Computer Components:Top Level View

Instruction Cycle• Two steps:

—Fetch—Execute

Fetch Cycle• Program Counter (PC) holds address of

next instruction to fetch• Processor fetches instruction from

memory location pointed to by PC• Increment PC

—Unless told otherwise

• Instruction loaded into Instruction Register (IR)

• Processor interprets instruction and performs required actions

Execute Cycle• Processor-memory

—data transfer between CPU and main memory

• Processor I/O—Data transfer between CPU and I/O module

• Data processing—Some arithmetic or logical operation on data

• Control—Alteration of sequence of operations—e.g. jump

• Combination of above

Example of Program Execution

Instruction Cycle State Diagram Interrupts• Mechanism by which other modules (e.g.

I/O) may interrupt normal sequence of processing

• Program—e.g. overflow, division by zero

• Timer—Generated by internal processor timer—Used in pre-emptive multi-tasking

• I/O—from I/O controller

• Hardware failure—e.g. memory parity error

Program Flow Control Interrupt Cycle• Added to instruction cycle• Processor checks for interrupt

—Indicated by an interrupt signal

• If no interrupt, fetch next instruction• If interrupt pending:

—Suspend execution of current program —Save context—Set PC to start address of interrupt handler

routine—Process interrupt—Restore context and continue interrupted

program

Transfer of Control via Interrupts Instruction Cycle with Interrupts

Program TimingShort I/O Wait

Program TimingLong I/O Wait

Instruction Cycle (with Interrupts) -State Diagram Multiple Interrupts

• Disable interrupts—Processor will ignore further interrupts whilst

processing one interrupt—Interrupts remain pending and are checked

after first interrupt has been processed—Interrupts handled in sequence as they occur

• Define priorities—Low priority interrupts can be interrupted by

higher priority interrupts—When higher priority interrupt has been

processed, processor returns to previous interrupt

Multiple Interrupts - Sequential Multiple Interrupts – Nested

Time Sequence of Multiple Interrupts Connecting• All the units must be connected• Different type of connection for different

type of unit—Memory—Input/Output—CPU

Computer Modules Memory Connection• Receives and sends data• Receives addresses (of locations)• Receives control signals

—Read—Write—Timing

Input/Output Connection(1)• Similar to memory from computer’s

viewpoint• Output

—Receive data from computer—Send data to peripheral

• Input—Receive data from peripheral—Send data to computer

Input/Output Connection(2)• Receive control signals from computer• Send control signals to peripherals

—e.g. spin disk

• Receive addresses from computer—e.g. port number to identify peripheral

• Send interrupt signals (control)

CPU Connection• Reads instruction and data• Writes out data (after processing)• Sends control signals to other units• Receives (& acts on) interrupts

Buses• There are a number of possible

interconnection systems• Single and multiple BUS structures are

most common• e.g. Control/Address/Data bus (PC)• e.g. Unibus (DEC-PDP)

What is a Bus?• A communication pathway connecting two

or more devices• Usually broadcast • Often grouped

—A number of channels in one bus—e.g. 32 bit data bus is 32 separate single bit

channels

• Power lines may not be shown

Data Bus• Carries data

—Remember that there is no difference between “data” and “instruction” at this level

• Width is a key determinant of performance—8, 16, 32, 64 bit

Address bus• Identify the source or destination of data• e.g. CPU needs to read an instruction

(data) from a given location in memory• Bus width determines maximum memory

capacity of system—e.g. 8080 has 16 bit address bus giving 64k

address space

Control Bus• Control and timing information

—Memory read/write signal—Interrupt request—Clock signals

Bus Interconnection Scheme Big and Yellow?• What do buses look like?

—Parallel lines on circuit boards—Ribbon cables—Strip connectors on mother boards

– e.g. PCI

—Sets of wires

Physical Realization of Bus Architecture Single Bus Problems• Lots of devices on one bus leads to:

—Propagation delays– Long data paths mean that co-ordination of bus use

can adversely affect performance– If aggregate data transfer approaches bus capacity

• Most systems use multiple buses to overcome these problems

Traditional (ISA)(with cache) High Performance Bus

Bus Types• Dedicated

—Separate data & address lines

• Multiplexed—Shared lines—Address valid or data valid control line—Advantage - fewer lines—Disadvantages

– More complex control– Ultimate performance

Bus Arbitration• More than one module controlling the bus• e.g. CPU and DMA controller• Only one module may control bus at one

time• Arbitration may be centralised or

distributed

Centralised or Distributed Arbitration• Centralised

—Single hardware device controlling bus access– Bus Controller– Arbiter

—May be part of CPU or separate

• Distributed—Each module may claim the bus—Control logic on all modules

Timing• Co-ordination of events on bus• Synchronous

—Events determined by clock signals—Control Bus includes clock line—A single 1-0 is a bus cycle—All devices can read clock line—Usually sync on leading edge—Usually a single cycle for an event

Synchronous Timing Diagram Asynchronous Timing – Read Diagram

Asynchronous Timing – Write Diagram PCI Bus• Peripheral Component Interconnection• Intel released to public domain• 32 or 64 bit• 50 lines

PCI Bus Lines (required)• Systems lines

—Including clock and reset

• Address & Data—32 time mux lines for address/data—Interrupt & validate lines

• Interface Control• Arbitration

—Not shared—Direct connection to PCI bus arbiter

• Error lines

PCI Bus Lines (Optional)• Interrupt lines

—Not shared

• Cache support• 64-bit Bus Extension

—Additional 32 lines—Time multiplexed—2 lines to enable devices to agree to use 64-

bit transfer

• JTAG/Boundary Scan—For testing procedures

PCI Commands• Transaction between initiator (master)

and target• Master claims bus• Determine type of transaction

—e.g. I/O read/write

• Address phase• One or more data phases

PCI Read Timing Diagram

PCI Bus Arbiter PCI Bus Arbitration

Foreground Reading• Stallings, chapter 3 (all of it)• www.pcguide.com/ref/mbsys/buses/

• In fact, read the whole site!• www.pcguide.com/